How many hours it takes?

the impacts on your damage and/or exp gain are even better.

Specifically, links that buff damage are
Hayato: +10 all-stats, 5 atk/matk (level 70)
Cannoneer: +15/25/35 all stats, 5/10/15% hp/mp (level 70/120/210)
Demon Slayer: 10/15/20% boss damage
Demon Avenger: 5/10/15% total damage
Phantom: 10/15/20% crit rate
Luminous: 10/15/20% ignore pdr
Mercedes: 10/15/20% exp gain
Xenon: 5/10% all stats (70/120)
Kanna: 5/10% total damage

If you're starting from 0, I'd probably prefer Demon Avenger, Xenon, Kanna, Mercedes. Any of those will make future characters quicker.

As for character cards, the more level 100+ chars the better, that's about it really. Until you have 3 sets of S cards don't worry about specific classes.